NOTE: There are two basic sizes of M4 Carbine receiver extensions (buffer tubes): mil-spec and commercial-spec. If you have a commercial-spec size extension on your rifle, make sure to order the Commercial-Spec MOE.If you have one of the uncommon military tubes with an extended slant-back, along with the Mil-Spec MOE you will also need to order the Extended Rubber Butt-Pad, 0.55" (sold separately).